1. Field of the Invention
The present invention relates to a tape substrate and a method for plating the tape substrate, and more particularly to a tape substrate and a fabricating method thereof, in which tin is plated on a pattern portion of the tape substrate where an electronic element is to be mounted, without formation of pores or whiskers.
2. Description of the Related Art
Generally, tape substrates are mainly applicable to foldable electronic appliances such as an LCD panel, a portable phone, or a notebook computer. In pace with miniaturization and lightness of such electronic appliances, various substrates have been developed. For example, a tape automated bonding (TAB) substrate, a tape ball grid array (TBGA) substrate, an application specific integrated circuit (ASIC) substrate, and a chip on film (COF) substrate have been developed.
FIG. 1 is a sectional view illustrating a conventional tape substrate. This tape substrate is fabricated by bonding a copper foil, about 20 μm thick, using bonding material 12, to an upper surface of an insulating film 10 made of a material such as polyimide or polyether sulphon, coating photoresist over an upper surface of the copper foil, and then exposing the photoresist to light by use of a mask having a pattern corresponding to a desired wiring pattern, thereby transferring the wiring pattern onto the photoresist.
After the transfer of the pattern, the copper foil is subjected to an etching process, so that it is patterned to have a desired pattern. Thereafter, the photoresist is removed. Thus, a copper foil pattern 20 is completely formed.
A solder resist 30 is coated on the tape substrate formed with the copper foil pattern 20, at a region other than a connecting area, so that the surface portion of the tape substrate corresponding to the connecting area is maintained in an exposed state. Thereafter, tin is plated on the exposed surface portion of the tape substrate to form a tin layer 27 adapted to prevent oxidation from occurring at the connecting area while enabling an electronic element to be mounted on the tape substrate at the connecting area.
The plated tin layer 27 exhibits a superior flexibility, while being less toxic to the human body. The tin layer 27 also exhibits a superior solderability because its melting point is low. The tin layer 27 can also be inexpensively formed, thereby reducing the fabricating cost of the tape substrate. Furthermore, it is possible to form the tin layer 27 to have a uniform thickness of 1 μm or less. The tin plating solution adapted to form the tin layer 27 can be easily handled. In addition, the tin layer 27 exhibits a high bonding strength to bumps made of gold. By virtue of such advantages, the application of such a plated tin layer is being widened.
In accordance with the above mentioned tape substrate and fabricating method thereof, however, an electroless thin plating solution may penetrate between the edge of the solder resist 30 and the copper foil pattern 20 during a process of plating thin on the tape substrate. As a result, a local cell is formed between the copper foil and the electroless tin plating solution, so that copper may be eluted from the copper foil pattern, thereby causing voids C to be formed at the copper foil pattern. Where such a void C has a large size, open-circuit fault may be generated at the copper foil pattern 20.
Due to the void C, a bending stress is generated at the tape substrate, so that the tape substrate may be deformed. In severe cases, the tape substrate may be bent, thereby causing its copper foil pattern to be damaged.
Typically, stress may be generated at the interface between the copper foil pattern and the tin layer plated thereon, due to the hetero structures of those layers. For this reason, needle-shaped precipitates called “whiskers” may grow for several days or even several years. Such whiskers may short-circuit adjacent pattern portions of the copper foil pattern, thereby causing circuit damage. However, there is no method capable of effectively suppressing such whiskers.
A heat treatment or alloying method has often been used to suppress formation of whiskers. However, the heat treatment method requires a lengthened process time, thereby causing a degradation in productivity. Furthermore, the tape substrate may be degraded under a certain heating condition, so that it may have a poor quality. As the tin layer 27 is heated, the tin component thereof is coupled with the copper element of the copper foil pattern, thereby forming a tin-copper alloy layer 23. In order to obtain a desired thickness of a pure tin layer portion in the tin layer 27, it is necessary to perform the tin plating process for a prolonged period of time. There is also a problem in that the tin-cooper alloy layer 23 has an excessively large thickness.
Where the above alloying is carried out to suppress formation of whiskers, there is a degradation in electrical characteristics and solderability. As a result, the mounting strength for electronic elements is weakened. Thus, this method cannot basically provide a solution to the above mentioned problems.